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Loads on Bridges

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A) Permanent Loads:

01. Dead Loads

02. Superimposed Dead Loads

03. Pressures (earth, water)

C) Deformation and Response Loads:

12. Creep

13. Shrinkage

14. Settlement

15. Uplift

16. Thermal Forces

B) Temporary Loads:

04. Vehicle Live Loads

05. Earthquake Forces

06. Wind Forces

07. Channel Forces

08. Longitudinal Forces

09. Centrifugal Forces

10. Impact Forces

11. Construction Loads

Types of loads on bridges:

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A- Permanent Loads:Permanent loads, are those loads which always remain and

act on a bridge throughout its life.

Permanent loads are divided into the following three major

categories.

1. Dead Load:

The dead load on a superstructure is the own weight of all

superstructure elements (i.e., those elements above the

bearings).

This would include, but are not be limited to:

The deck, wearing surface, sidewalks and railings, parapets,

primary members, secondary members (including all bracing,

connection plates, etc.), stiffeners, signing, and utilities.

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2.402 ton/m3

7.85 ton/m3

3.204 ton/m3

0.379 Kg/m2

1.9 ton/m3

1.602 ton/m3

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2. Superimposed Dead Load:

In composite construction superimposed dead loads are

those loads placed on the superstructure after the deck

has cured and begun to work with the primary

members in resisting loads.

From the list of elements above, the designer would

separate items such as sidewalks, railings, parapets, signing,

utilities, and the wearing surface.

Superimposed dead load is part of the total dead load. It is

separated from the rest of the dead loads because it is

resisted by a composite section, therefore cause less

deflection and stress in the stringer than other dead loads.

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3. Pressures:

Pressures due to earth or water are also considered

permanent loads.

These loads primarily affect substructure elements.

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B- Temporary Loads:

Temporary loads are those loads which are placed on a

bridge for only a short period of time.

There are several classes of temporary loads which the

designer must consider.

Live loads represent the major temporary loading condition.

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The term live load means a load that moves along the

length of a span.

The principal loading constraint which highway bridges are

designed by is truck loading.

To give designers the ability to accurately model the live

load on a structure, hypothetical truck classes designated

as H (two axels) and HS (three axels) class trucks

were developed by AASHTO.

4. Vehicle Live Load:

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H and HS trucks do not represent an actual truck being

used to transport goods and materials. They are

approximations used to simulate the greatest bending

and shear forces caused by actual trucks.

4. Vehicle Live Load:

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In 1935, what was then called AASHO issued a loading

scheme based on a train of trucks. These are identified as

(H-20-35) and (H-15-35) shown in Figure 3.11

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To meet the demands of heavier trucks, the introduction of

five new truck classes was made in 1944.

These classes have the following designations and gross vehicle

weights:

H10-44 (20,000 lb - 89 KN)

H15-44 (30,000 lb - 133 KN)

H20-44 (40,000 lb - 178 KN)

HS15-44 (54,000 lb - 240 KN)

HS20-44 (72,000 lb - 320 KN)

Today, all but the H10-44 vehicle are still included in the

AASHTO standard specifications.

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The loading is performed by placing one truck, per

lane, per span.

The truck is moved along the span, to determine the

point where it produces the maximum moment.

The HS trucks have a variable spacing between the two

rear axles.

Vehicle Live Load according to ASSHTO1. Truck Load

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• This distance between axles, varying from 14 to 30 ft

(4.27 to 9.14 m), is used to create a live loading situation

which will induce maximum moment in a span.

• For simply supported bridges, this value will be the

14 ft minimum.

• In continuous spans, the distance between axles is

varied to position the axles at adjacent supports in such

a fashion as to create the maximum negative moment

Vehicle Live Load according to ASSHTO

1. Truck Load

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Consists from:a) Uniformly distributed load

b) Concentrated load (Varies for moment and shear

computations).

• The concentrated load is moved along the span to

determine the point of maximum moment.

• To determine the maximum positive moment in

continuous spans, only one concentrated load is used

(which is also true for a simple span bridge).

• To determine the maximum negative moment in a

continuous span, two concentrated loads are used.

Vehicle Live Load according to ASSHTO

2. Lane Load

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In the AASHTO LRFD Specifications, which is gradually

replacing the AASHTO Standard Specifications, HL-93

live load is used.

HL-93 consists of a design truck or tandem (whichever

produces the greater forces), combined with a design

lane load.

The design truck is identical to HS20-44.

The design tandem consists of a pair of 25,000 lb (111

KN) axles spaced 4.0 ft (1.2 m) apart.

The design lane load in the AASHTO LRFD

Specifications is 0.64 kip/ft (9.34 KN/m). This load is

used in conjunction with the design truck or tandem.

Vehicle Live Load according to ASSHTO LRFD

HL-93 Model

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HL-93 Model

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12

.5 K

ips

12

.5 K

ips

12

.5 K

ips

12

.5 K

ips

HL-93 Model

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The AASHTO HL-93 design loads:

a) Design truck plus design lane.

b) Design tandem plus design lane.

HL-93 Model

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A reduction of the live load is permitted for bridges

with three or more lanes, that have maximum stress

caused by fully loading each lane.

A reduction to 90% is allowed for three lane structures

and to 75% for bridges with four or more lanes.

Reduction is justified on the premise that it is

unlikely that all the lanes will be fully loaded to the

maximum at the same time.

Reduction of the Live Load (AASHTO Standard)

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8. Longitudinal Forces: Longitudinal forces is also called Braking Force.

As a truck brakes, the load of the vehicle is transferred

from the truck wheels to the bridge deck.

AASHTO Standard Specifications specifies that 5 percent

of the appropriate lane load along with the

concentrated force for moment, to be used as the

resulting longitudinal force.

This force is applied 6 ft (1.8 m) above the top of deck

surface.

The breaking force is resisted by the piers and/or

abutments which support fixed bearings.

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10. Impact (Dynamic Load Allowance):In order to account for the dynamic effects of a vehicle

riding over a structure, an impact factor is used as a multiplier

for certain structural elements.

AASHTO Standard Specification defines the impact factor

as follows:

Live load forces are then multiplied by this factor.

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The impact factor applies only for certain elements which

AASHTO Standard Specifications classifies as Group A and

Group B.

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The length of span loaded L also varies depending on the type

of element being analyzed.

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C) Deformation and Response Loads:Deformation loads are those loads induced by the internal or

external change in material properties or member geometry.

Creep

Shrinkage

Support settlement

Response loads are those loads created by the response of the

structure to a given loading condition. Uplift is an example of

a response load.

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12. Creep:Creep is the deformation of concrete caused by loads sustained

over a period of time.

Creep deformations taking place in the elastic range are

approximately proportional to the applied stress.

Once any member is loaded, it deforms elastically. Concrete,

however, will continue to deform over an extended period of

time.

One way of counteracting the effects of creep is to simply use

a higher strength concrete with low water/ cement ratio.

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With respect to highway bridges, creep can cause changes in

the physical length of concrete members.

This deformation can lead to problems with bearing

alignment and superstructure stability.

The ACI Code instructs designers to make a “realistic

assessment” of the effects of creep in computing the ultimate

deformation of a concrete structure.

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13. Shrinkage:Shrinkage is the natural change in volume of concrete

caused by a moisture loss while drying.

When shrinkage takes place, the concrete volume generally

decreases (shrinks).

Shrinkage is sensitive to the water/cement ratio of the

concrete, and the humidity condition of the air.

Reinforcement is added perpendicular to the main

reinforcement to account for tensile stresses induced by

shrinkage.

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14. Settlement:Settlement is the downward movement of a footing, pavement, or

structure due to deformations of the supporting soil or piles.

Settlement can be initiated by a number of factors which

include, but are not limited to:

•Overloading the supporting soil or piles

•Lowering the water table for spread footings or friction piles

•Vibrations from live loads or seismic loads

•Changes in soil properties

Of particular concern to the bridge design are differential

settlements where a foundation will move downward in an

uneven fashion.

Such settlements can result in cracking of substructure elements

and instability at superstructure joints and support points.

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15. Uplift:For a continuous span structure, different loads can combine

in such a fashion that results in the superstructure being

lifted upward from the substructure supports. Such a

phenomenon is known as uplift.

When uplift is possible, AASHTO requires that

superstructures be designed with appropriate superstructure

restraining measures (tension ties).

Uplift typically occurs in continuous span bridges with widely

varying adjacent span lengths (e.g., a long span next to a

short end span).

Design Methods

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AASHTO were based on the Allowable Stress Design

(ASD) philosophy until 1970, after which the Load Factor

Design (LFD) was incorporated in the specifications.

AASHTO introduced the AASHTO LRFD Bridge Design

Specifications in 1994 .

LRFD based on probability based limit state philosophy.

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Group Loading Combinations Loads do not act individually but in various

combinations.

The probability of all worst loads acting on the structure

simultaneously is very small, so engineers should not

simply add all the worst case loads together to design a

bridge.

AASHTO developed a set of loading combinations which

are divided into various groups and represent probable

occurring combinations of loads on a structure.

Different expressions have been used by AASHTO and

AASHTO LRFD Specifications to reflect the difference in

their design philosophies.

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1. AASHTO Standard Specifications.

The general equation used to define

a group load is given by:

The subscript values in this Equation

represent different types of loads.

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AASHTO Standard Specifications values for γ and β

based on the working stress (service load) design

method.

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AASHTO Standard Specifications values for γ and β

based on load factor (limit state) design method.

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The coefficient β varies based on the type of load.

The load factor γ is unity for all working stress groupings and

varies only for load factor design groupings.

The last column of the Table 3.2 gives a specified increase of

allowable stresses for the working stress design method.

From Table 3.3 the β coefficients for earth pressure and dead

load vary depending on the load group and design method.βd = 1.0 for flexural and tension members

βE = 1.3 for horizontal earth pressure on retaining walls.

βE = 1.0 for vertical earth pressure.

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Earth and Dead Load coefficients

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Group Loading (Load Factor Design)

Group I: LF = 1.3[βd D+1.67 (L+I)]

Group II: LF = 1.3[βd D+W]

Group III: LF = 1.3[βd D+L+I+0.3W+WL+LF]

Group IV: LF = 1.3[βd D+L+I+T]

Group V: LF = 1.25[βd D+W+T]

Group VI: LF = 1.25[βd D+L+I +0.3W+WL+LF+T]

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AASHTO LRFD is a probability based design method.

The philosophy is to have a relative constant probability of

structural failure for all structures and elements during their design

lives regardless of their types, geometry, materials, or construction

methods.

The measure of safety of any structure member is a function of

variability of loads and resistance. The bigger the variation of a

load, the larger the load factor should be (i.e. load factor for live

load should be larger than that of dead load).

Also, the more uncertainly of a material’s load resistance, the

smaller resistance factor it should have, so that all materials will

have similar factor of safety.

2. AASHTO LRFD Specifications.

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To achieve that design objective, strength limit states, service

limit states, and fatigue limit states are checked for every

structure member.

Strength limit states are intended to ensure that structures

have sufficient strength and stability under various load

conditions.

Service limit states are used to control deflection, crack

width, stress level, and stability under normal service

conditions.

Fatigue limit states are restrictions on stress range under

service loading to prevent fatigue failure during the design life

of the bridge.

2. AASHTO LRFD Specifications.

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Under each load combination, the total factored force effect

should be taken as:

2. AASHTO LRFD Specifications.

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Each member and connection of a structure should satisfy

the following limit states:

Strength I: This is a basic load combination relating to the normal

vehicular use of the structure without wind or any extreme

event loads such as earthquake. Most superstructure members are

controlled by this load combination.

Strength II: This load combination is used for owner-specified

special design vehicles or permit vehicles. Like Strength I load

combination, no wind or any extreme event load need to be

considered, so this load combination is not commonly used.

Strength III: This load combination relates to the bridge being

exposed to maximum wind velocity. Under such event, no live

load is assumed to be present on the bridge.

2. AASHTO LRFD Specifications

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Strength IV:

•This load combination is used for structures with very high

dead to live load force ratios.

•It may become controlling load combination if the structure

has a short span length and/or a large dead load.

Strength V:

•This load combination relates to normal vehicular use of the

bridge with wind velocity of 55 mph (90 km/h).

•When live load and wind loads are combined, both values are

reduced because the probability is very low for a structure to

experience a very heavy live load and extremely high wind load.

2. AASHTO LRFD Specifications.

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Extreme Event I:

This is a load combination related to earthquake.

Note that live load shall be considered based on daily traffic

volume of the bridge. For normal bridges, a live load factor of

0.5 may be used, which indicates a low probability of the

presence of maximum live load at the time when a large

earthquake may occur.

Extreme Event II:

This load combination is used for extreme events such as ice

load, collision by vessels and vehicles.

Only one of such events should be considered at a time.

Like in an earthquake event, only reduced live load need to be

considered at these extreme events.

2. AASHTO LRFD Specifications.

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Service I:

This load combination is used for normal operational use of

the bridge with a 55 mph (90 km/h) wind.

All loads are taken at their nominal values and extreme loads are

excluded.

This load combination is used to control deflection, crack width

in reinforced concrete structures, compressive stress in pre-

stressed concrete members, and soil slope stability.

Service II.

This load combination is for preventing yielding of steel

structures due to vehicular live load.

The live load used in this load combination is approximately

halfway between that used for service I and Strength I limit

states.

2. AASHTO LRFD Specifications.

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Service III:

This load combination relates only to tension in prestressed

concrete superstructure.

Researchers have found that if nominal design live load is used,

the superstructure will be overdesigned for concrete tensile

stress. Therefore, a load factor of 0.80 is applied to the live load

in this load combination.

Service IV:

This load combination relates only to tension in prestressed

concrete substructure to control cracks. The 0.70 factor on

wind represents a wind velocity of 84 mph (135 km/h), which

reflects the probability that the prestressed concrete substructure

will be subjected to a tensile stress once in every 10 years.

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Fatigue:

Fatigue and fracture load combination relates to repetitive

gravitational vehicular live load and dynamic responses.

The live load factor of 0.75 reflects a load level that represents

the majority of truck population.

Note that only a single truck with a constant spacing of 30 feet

(9.1 m) between the 32 kip (142 kN) axles should be applied for

this load combination (AASHTO LRFD 3.6.1.4.1).

2. AASHTO LRFD Specifications.

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